On Fri, Dec 22, 2017 at 10:14:43PM +0800, Huang, Ying wrote: > Minchan Kim <minchan@xxxxxxxxxx> writes: > > > On Thu, Dec 21, 2017 at 03:48:56PM +0800, Huang, Ying wrote: > >> Minchan Kim <minchan@xxxxxxxxxx> writes: > >> > >> > On Wed, Dec 20, 2017 at 09:26:32AM +0800, Huang, Ying wrote: > >> >> From: Huang Ying <ying.huang@xxxxxxxxx> > >> >> > >> >> When the swapin is performed, after getting the swap entry information > >> >> from the page table, system will swap in the swap entry, without any > >> >> lock held to prevent the swap device from being swapoff. This may > >> >> cause the race like below, > >> >> > >> >> CPU 1 CPU 2 > >> >> ----- ----- > >> >> do_swap_page > >> >> swapin_readahead > >> >> __read_swap_cache_async > >> >> swapoff swapcache_prepare > >> >> p->swap_map = NULL __swap_duplicate > >> >> p->swap_map[?] /* !!! NULL pointer access */ > >> >> > >> >> Because swapoff is usually done when system shutdown only, the race > >> >> may not hit many people in practice. But it is still a race need to > >> >> be fixed. > >> >> > >> >> To fix the race, get_swap_device() is added to check whether the > >> >> specified swap entry is valid in its swap device. If so, it will keep > >> >> the swap entry valid via preventing the swap device from being > >> >> swapoff, until put_swap_device() is called. > >> >> > >> >> Because swapoff() is very race code path, to make the normal path runs > >> >> as fast as possible, RCU instead of reference count is used to > >> >> implement get/put_swap_device(). From get_swap_device() to > >> >> put_swap_device(), the RCU read lock is held, so synchronize_rcu() in > >> >> swapoff() will wait until put_swap_device() is called. > >> >> > >> >> In addition to swap_map, cluster_info, etc. data structure in the > >> >> struct swap_info_struct, the swap cache radix tree will be freed after > >> >> swapoff, so this patch fixes the race between swap cache looking up > >> >> and swapoff too. > >> >> > >> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx> > >> >> Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > >> >> Cc: Minchan Kim <minchan@xxxxxxxxxx> > >> >> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > >> >> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> > >> >> Cc: Shaohua Li <shli@xxxxxx> > >> >> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > >> >> Cc: "Jrme Glisse" <jglisse@xxxxxxxxxx> > >> >> Cc: Michal Hocko <mhocko@xxxxxxxx> > >> >> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > >> >> Cc: David Rientjes <rientjes@xxxxxxxxxx> > >> >> Cc: Rik van Riel <riel@xxxxxxxxxx> > >> >> Cc: Jan Kara <jack@xxxxxxx> > >> >> Cc: Dave Jiang <dave.jiang@xxxxxxxxx> > >> >> Cc: Aaron Lu <aaron.lu@xxxxxxxxx> > >> >> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx> > >> >> > >> >> Changelog: > >> >> > >> >> v4: > >> >> > >> >> - Use synchronize_rcu() in enable_swap_info() to reduce overhead of > >> >> normal paths further. > >> > > >> > Hi Huang, > >> > >> Hi, Minchan, > >> > >> > This version is much better than old. To me, it's due to not rcu, > >> > srcu, refcount thing but it adds swap device dependency(i.e., get/put) > >> > into every swap related functions so users who don't interested on swap > >> > don't need to care of it. Good. > >> > > >> > The problem is caused by freeing by swap related-data structure > >> > *dynamically* while old swap logic was based on static data > >> > structure(i.e., never freed and the verify it's stale). > >> > So, I reviewed some places where use PageSwapCache and swp_entry_t > >> > which could make access of swap related data structures. > >> > > >> > A example is __isolate_lru_page > >> > > >> > It calls page_mapping to get a address_space. > >> > What happens if the page is on SwapCache and raced with swapoff? > >> > The mapping got could be disappeared by the race. Right? > >> > >> Yes. We should think about that. Considering the file cache pages, the > >> address_space backing the file cache pages may be freed dynamically too. > >> So to use page_mapping() return value for the file cache pages, some > >> kind of locking is needed to guarantee the address_space isn't freed > >> under us. Page may be locked, or under writeback, or some other locks > > > > I didn't look at the code in detail but I guess every file page should > > be freed before the address space destruction and page_lock/lru_lock makes > > the work safe, I guess. So, it wouldn't be a problem. > > > > However, in case of swapoff, it doesn't remove pages from LRU list > > so there is no lock to prevent the race at this moment. :( > > Take a look at file cache pages and file cache address_space freeing > code path. It appears that similar situation is possible for them too. > > The file cache pages will be delete from file cache address_space before > address_space (embedded in inode) is freed. But they will be deleted > from LRU list only when its refcount dropped to zero, please take a look > at put_page() and release_pages(). While address_space will be freed > after putting reference to all file cache pages. If someone holds a > reference to a file cache page for quite long time, it is possible for a > file cache page to be in LRU list after the inode/address_space is > freed. > > And I found inode/address_space is freed witch call_rcu(). I don't know > whether this is related to page_mapping(). > > This is just my understanding. > > >> need to be held, for example, page table lock, or lru_lock, etc. For > >> __isolate_lru_page(), lru_lock will be held when it is called. And we > >> will call synchronize_rcu() between clear PageSwapCache and free swap > >> cache, so the usage of swap cache in __isolate_lru_page() should be > >> safe. Do you think my analysis makes sense? > > > > I don't understand how synchronize_rcu closes the race with spin_lock. > > Paul might help it. > > Per my understanding, spin_lock() will preempt_disable(), so > synchronize_rcu() will wait until spin_unlock() is called. Only when CONFIG_PREEMPT=n! In CONFIG_PREEMPT=y kernels, preempt_disable() won't necessarily prevent synchronize_rcu() from completing. Now, preempt_disable() does prevent synchronize_sched() from completing, but that would require changing the rcu_read_lock() and rcu_read_unlock() to rcu_read_lock_sched()/rcu_read_unlock_sched() or preempt_enable()/preempt_disable(). Another fix would be to invoke rcu_read_lock() just after acquiring the spinlock and rcu_read_unlock() just before releasing it. Thanx, Paul > > Even if we solve it, there is a other problem I spot. > > When I see migrate_vma_pages, it pass mapping to migrate_page which > > accesses mapping->tree_lock unconditionally even though the address_space > > is already gone. > > Before migrate_vma_pages() is called, migrate_vma_prepare() is called, > where pages are locked. So it is safe. > > > Hmm, I didn't check all sites where uses PageSwapCache, swp_entry_t > > but gut feeling is it would be not simple. > > Yes. We should check all sites. Thanks for your help! > > Best Regards, > Huang, Ying > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
